The Chinese government and the company that supplied a contaminated ingredient are slowing the federal investigation into the nationwide recall of pet food, a U.S. Food and Drug Administration official said Tuesday.

Researchers, however, are making strides toward uncovering what has sickened cats and dogs nationwide. A lead scientist said yesterday he is convinced a second contaminant was in the wheat gluten, which FDA and independent researchers said was laced with high amounts of melamine, a chemical used in plastics.

Dr. Richard Goldstein, associate professor of medicine at Cornell University's College of Veterinary Medicine and a kidney specialist who is researching the outbreak's health impact on pets, said he and other researchers saw what they believe is a second contaminant in the gluten and the urine of infected animals, but have yet to identify it. Cornell is among labs working with the FDA.

"The concerted effort now is to identify what else is in there, and what's in the crystals" of infected animals' urine and tissue, Goldstein said.

Michael Rogers, director of the FDA's field investigations division, told the Pittsburgh Tribune-Review the agency has asked the Chinese government for help investigating the gluten and the supplier, Xuzhou Anying Biologic Technology Development Co. Ltd., based in Jiangsu province.

The FDA is disappointed with slow and incomplete Chinese responses, Rogers said.

"I usually don't speak in terms of cooperative or not cooperative," he said.

Chu Maoming, spokesman for the Chinese Embassy in Washington D.C., did not return calls or an e-mail requesting comment.

He told the Trib on March 30: "The Chinese Embassy is working closely with the FDA officers to determine the real cause." Since then, he has declined repeated requests for interviews with the embassy representative working with the FDA.

Although the agency got some information from the Chinese, Rogers said, "There remain a number of questions."

Federal investigators haven't determined whether Xuzhou Anying shipped other food products to the United States, or what other Chinese companies it sold wheat gluten to that, in turn, might have been shipped here, Rogers said.

Xuzhou Anying's Web site said it also exports carrots, garlic, ginger, corn protein powder, vegetables and feed. Rogers said Chinese officials have not responded to the U.S. government's question about whether any products other than wheat gluten were shipped here.

"We're certainly reviewing all products from this source," he said. Since the recall, the company has shipped only wheat gluten to the United States, but U.S. officials still are unsure what might have been shipped prior to the recall, Rogers said.

"From an operational standpoint, we still have questions about this company," he said.

The FDA is screening all wheat gluten imported from China and the Netherlands at U.S. ports and seizing all wheat gluten from Xuzhou Anying.

Under the microscope and even to the naked eye, the contaminated gluten looks different from uncontaminated samples, Goldstein said. Researchers see melamine granules and other colored granules throughout the gluten, he said.

"There appears to be other things in there, other than the melamine, but identifying what they are is a long process," he said.

He said researchers ruled out aminopterin -- used as rat poison in other countries -- which New York state officials previously announced was in the pet food.

The FDA, Cornell and other researchers found melamine in high concentrations in the gluten -- up to 6.6 percent of the product.

Even so, they do not believe the melamine made the animals sick, although they said it is a marker for tracking the outbreak, because the crystal found in the melamine and in animals' urine and tissue is distinctive to this outbreak.

Because of a dearth of past studies on melamine exposure in dogs and cats, the only way to know for sure if it could cause the outbreak would be to feed the compound to those animals, Goldstein said, adding, "That's not an option."

More than 10 laboratories are researching the crystals and working together to develop criteria to determine which kidney illnesses were caused by the contaminated pet food. Although the link is relatively easy to establish because of the distinctive crystals, the process needed to find them is expensive and time-consuming, Goldstein said.